BITS, BYTES, AND INTEGERS · PDF fileRepresenting information as bits Bit-level manipulations...

BITS, BYTES, AND INTEGERS

COMPUTER ARCHITECTURE AND ORGANIZATION

22

Today: Bits, Bytes, and Integers

Representing information as bits Bit-level manipulations Integers

Representation: unsigned and signed Conversion, casting Expanding, truncating Addition, negation, multiplication, shifting

Making ints from bytes Summary

33

Encoding Byte Values

Byte = 8 bits Binary 000000002 to 111111112

Decimal: 010 to 25510

Hexadecimal 0016 to FF16

Base 16 number representation Use characters ‘0’ to ‘9’ and ‘A’ to ‘F’Write FA1D37B16 in C as

0xFA1D37B 0xfa1d37b

0 0 00001 1 00012 2 00103 3 00114 4 01005 5 01016 6 01107 7 01118 8 10009 9 1001A 10 1010B 11 1011C 12 1100D 13 1101E 14 1110F 15 1111

44

Boolean Algebra

Developed by George Boole in 19th Century Algebraic representation of logic Encode “True” as 1 and “False” as 0

And A&B = 1 when both A=1 and B=1

Or A|B = 1 when either A=1 or B=1

Not ~A = 1 when A=0

Exclusive-Or (Xor) A^B = 1 when either A=1 or B=1, but not both

55

General Boolean Algebras

Operate on Bit Vectors Operations applied bitwise

All of the Properties of Boolean Algebra Apply

01101001& 0101010101000001

01101001| 0101010101111101

01101001^ 0101010100111100

~ 010101011010101001000001 01111101 00111100 10101010

66

Bit-Level Operations in C

Operations &, |, ~, ^ Available in C Apply to any “integral” data type

long, int, short, char, unsigned View arguments as bit vectors Arguments applied bit-wise

Examples (Char data type [1 byte]) In gdb, p/t 0xE prints 1110 ~0x41 → 0xBE

~010000012 → 101111102

~0x00 → 0xFF ~000000002 → 111111112

0x69 & 0x55 → 0x41 011010012 & 010101012 → 010000012

0x69 | 0x55 → 0x7D 011010012 | 010101012 → 011111012

77

Representing & Manipulating Sets

Representation Width w bit vector represents subsets of {0, …, w–1} aj = 1 if j ∈ A

01101001 { 0, 3, 5, 6 } 76543210 MSB Least significant bit (LSB)

01010101 { 0, 2, 4, 6 } 76543210

Operations & Intersection 01000001 { 0, 6 } | Union 01111101 { 0, 2, 3, 4, 5, 6 } ^ Symmetric difference 00111100 { 2, 3, 4, 5 } ~ Complement 10101010 { 1, 3, 5, 7 }

88

Contrast: Logic Operations in C

Contrast to Logical Operators &&, ||, ! View 0 as “False” Anything nonzero as “True” Always return 0 or 1 Short circuit

Examples (char data type) !0x41 → 0x00 !0x00 → 0x01 !!0x41 → 0x01

0x69 && 0x55 → 0x01 0x69 || 0x55 → 0x01 p && *p (avoids null pointer access)

99

Shift Operations

Left Shift: x << y Shift bit-vector x left y positions

Throw away extra bits on left Fill with 0’s on right

Right Shift: x >> y Shift bit-vector x right y positions Throw away extra bits on right

Logical shift Fill with 0’s on left

Arithmetic shift Replicate most significant bit on left

Undefined Behavior Shift amount < 0 or ≥ word size

01100010Argument x

00010000<< 3

00011000Log. >> 2

00011000Arith. >> 2

10100010Argument x

00010000<< 3

00101000Log. >> 2

11101000Arith. >> 2

0001000000010000

0001100000011000

0001100000011000

00010000

00101000

11101000

00010000

00101000

11101000

1010





1111

Data Representations

C Data Type Typical 32-bit Intel IA32 x86-64

char 1 1 1

short 2 2 2

int 4 4 4

long 4 4 8

long long 8 8 8

float 4 4 4

double 8 8 8

long double 8 10/12 10/16

pointer 4 4 8

1212

How to encode unsigned integers?

Just use exponential notation (4 bit numbers) 0110 = 0*23 + 1*22 + 1*21 + 0*20 = 6 1001 = 1*23 + 0*22 + 0*21 + 1*20 = 9 (Just like 13 = 1*101 + 3*100)

No negative numbers, a single zero (0000)

What happens if we represent positive&negativenumbers as an unsigned number plus sign bit?

1313

How to encode signed integers?

Want: Positive and negative values Want: Single circuit to add positive and negative

values (i.e., no subtractor circuit) Solution: Two’s complement Positive numbers easy (4 bits)

0110 = 0*23 + 1*22 + 1*21 + 0*20 = 6

Negative numbers a bit weird 1 + -1 = 0, so 0001 + X = 0, so X = 1111 -1 = 1111 in two’s compliment

1414

Unsigned & Signed Numeric Values

Equivalence Same encodings for

nonnegative values Uniqueness

Every bit pattern represents unique integer value

Each representable integer has unique bit encoding

⇒ Can Invert Mappings U2B(x) = B2U-1(x) Bit pattern for unsigned integer

T2B(x) = B2T-1(x) Bit pattern for two’s comp

integer

X B2T(X)B2U(X)0000 00001 10010 20011 30100 40101 50110 60111 7

–88–79–610–511–412–313–214–115

10001001101010111100110111101111

01234567

1515

Encoding Integers

C short 2 bytes long

Sign Bit For 2’s complement, most significant bit indicates sign 0 for nonnegative 1 for negative

short int x = 15213;short int y = -15213;

B2T (X ) = −xw−1 ⋅2w−1 + xi ⋅2 i

i=0

w−2

∑B2U(X ) = xi ⋅2 i

i=0

w−1

∑Unsigned Two’s Complement

SignBit

Decimal Hex Binary x 15213 3B 6D 00111011 01101101 y -15213 C4 93 11000100 10010011

1616

Encoding Example (Cont.)x = 15213: 00111011 01101101y = -15213: 11000100 10010011

Weight 15213 -15213 1 1 1 1 1 2 0 0 1 2 4 1 4 0 0 8 1 8 0 0

16 0 0 1 16 32 1 32 0 0 64 1 64 0 0

128 0 0 1 128 256 1 256 0 0 512 1 512 0 0

1024 0 0 1 1024 2048 1 2048 0 0 4096 1 4096 0 0 8192 1 8192 0 0

16384 0 0 1 16384 -32768 0 0 1 -32768

Sum 15213 -15213

1717

Numeric Ranges

Unsigned Values

UMin = 0000…0

UMax = 2w – 1111…1

Two’s Complement Values

TMin = –2w–1

100…0

TMax = 2w–1 – 1011…1

Other Values

Minus 1111…1

Decimal Hex Binary UMax 65535 FF FF 11111111 11111111 TMax 32767 7F FF 01111111 11111111 TMin -32768 80 00 10000000 00000000 -1 -1 FF FF 11111111 11111111 0 0 00 00 00000000 00000000

Values for W = 16

1818

Values for Different Word Sizes

Observations |TMin | = TMax + 1 Asymmetric range

UMax = 2 * TMax + 1

W 8 16 32 64

UMax 255 65,535 4,294,967,295 18,446,744,073,709,551,615 TMax 127 32,767 2,147,483,647 9,223,372,036,854,775,807 TMin -128 -32,768 -2,147,483,648 -9,223,372,036,854,775,808

C Programming #include <limits.h> Declares constants, e.g., ULONG_MAX LONG_MAX LONG_MIN

Values platform specific

1919





2020

T2UT2B B2U

Two’s Complement Unsigned

Maintain Same Bit Pattern

x uxX

Mapping Between Signed & Unsigned

Mappings between unsigned and two’s complement numbers:keep bit representations and reinterpret

U2TU2B B2T

Two’s ComplementUnsigned

Maintain Same Bit Pattern

ux xX

2121

Mapping Signed ↔ UnsignedSigned

0

1

2

3

4

5

6

7

-8

-7

-6

-5

-4

-3

-2

-1

Unsigned

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Bits

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

1010

1011

1100

1101

1110

1111

U2TT2U

2222

Mapping Signed ↔ UnsignedSigned

0

1

2

3

4

5

6

7

-8

-7

-6

-5

-4

-3

-2

-1

Unsigned

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Bits

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

1010

1011

1100

1101

1110

1111

=

+/- 16

2424

0

TMax

TMin

–1–2

0

UMaxUMax – 1

TMaxTMax + 1

2’s Complement Range

UnsignedRange

Conversion Visualized

2’s Comp. → Unsigned Ordering Inversion Negative → Big Positive

2525

Negation: Complement & Increment

Claim: Following Holds for 2’s Complement~x + 1 == -x

Complement Observation: ~x + x == 1111…111 == -1

1 0 0 1 0 11 1x

0 1 1 0 1 00 0~x+

1 1 1 1 1 11 1-1

2626

Complement & Increment Examples

Decimal Hex Binary x 15213 3B 6D 00111011 01101101 ~x -15214 C4 92 11000100 10010010 ~x+1 -15213 C4 93 11000100 10010011 y -15213 C4 93 11000100 10010011

x = 15213

Decimal Hex Binary 0 0 00 00 00000000 00000000 ~0 -1 FF FF 11111111 11111111 ~0+1 0 00 00 00000000 00000000

x = 0

2727

Signed vs. Unsigned in C

Constants By default are considered to be signed integers Unsigned if have “U” as suffix0U, 4294967259U

Casting Explicit casting between signed & unsigned same as U2T and T2U

int tx, ty;unsigned ux, uy;tx = (int) ux;uy = (unsigned) ty;

Implicit casting also occurs via assignments and procedure callstx = ux;uy = ty;

2828

0 0U == unsigned-1 0 < signed-1 0U > unsigned2147483647 -2147483648 > signed2147483647U -2147483648 < unsigned-1 -2 > signed(unsigned) -1 -2 > unsigned2147483647 2147483648U < unsigned

> signed

Casting Surprises Expression Evaluation

If there is a mix of unsigned and signed in single expression, signed values implicitly cast to unsigned

Including comparison operations <, >, ==, <=, >=

Constant1 Constant2 Relation Evaluation0 0U-1 0-1 0U2147483647 -2147483647-1 2147483647U -2147483647-1 -1 -2 (unsigned)-1 -2 2147483647 2147483648U

2147483647 (int) 2147483648U

2929

Code Security Example

Similar to code found in FreeBSD’s implementation of getpeername

There are legions of smart people trying to find vulnerabilities in programs

/* Kernel memory region holding user-accessible data */#define KSIZE 1024char kbuf[KSIZE];

/* Copy at most maxlen bytes from kernel region to user buffer */int copy_from_kernel(void *user_dest, int maxlen) {

/* Byte count len is minimum of buffer size and maxlen */int len = KSIZE < maxlen ? KSIZE : maxlen;memcpy(user_dest, kbuf, len);return len;

}

3030

Typical Usage




}

#define MSIZE 528

void getstuff() {char mybuf[MSIZE];copy_from_kernel(mybuf, MSIZE);printf(“%s\n”, mybuf);

}

3131

Malicious Usage




}

#define MSIZE 528

void getstuff() {char mybuf[MSIZE];copy_from_kernel(mybuf, -MSIZE);. . .

}

/* Declaration of library function memcpy */void *memcpy(void *dest, void *src, size_t n);

3232

SummaryCasting Signed ↔ Unsigned: Basic Rules

Bit pattern is maintained But reinterpreted Can have unexpected effects: adding or

subtracting 2w

Expression containing signed and unsigned int int is cast to unsigned!!

3333





3434

Sign Extension

Task: Given w-bit signed integer x Convert it to w+k-bit integer with same value

Rule: Make k copies of sign bit: X ′ = xw–1 ,…, xw–1 , xw–1 , xw–2 ,…, x0

k copies of MSB• • •X

X′

• • • • • •

• • •

w

wk

3535

Sign Extension Example

Converting from smaller to larger integer data type C automatically performs sign extension

short int x = 15213;int ix = (int) x; short int y = -15213;int iy = (int) y;

Decimal Hex Binaryx 15213 3B 6D 00111011 01101101ix 15213 00 00 3B 6D 00000000 00000000 00111011 01101101y -15213 C4 93 11000100 10010011iy -15213 FF FF C4 93 11111111 11111111 11000100 10010011

3636

Summary:Expanding, Truncating: Basic Rules

Expanding (e.g., short int to int) Unsigned: zeros added Signed: sign extension Both yield expected result

Truncating (e.g., unsigned to unsigned short) Unsigned/signed: bits are truncated Result reinterpreted Unsigned: mod operation Signed: similar to mod For small numbers yields expected behaviour

3737




Summary

3838

Unsigned Addition

Standard Addition Function Ignores carry output

Implements Modular Arithmetics = UAddw(u , v) = u + v mod 2w

UAdd w(u,v) =u + v u + v < 2w

u + v − 2w u + v ≥ 2w

• • •• • •

uv+

• • •u + v• • •

True Sum: w+1 bits

Operands: w bits

Discard Carry: w bits UAddw(u , v)

3939

0 2 4 6 8 10 12 140

2

4

68

1012

14

0

4

8

12

16

20

24

28

32

Integer Addition

Visualizing (Mathematical) Integer Addition

Integer Addition4-bit integers u, vCompute true sum

Add4(u , v)Values increase

linearly with u and vForms planar surface

Add4(u , v)

u

v

4040

0 2 4 6 8 10 12 140

2

4

68

1012

14

0

2

4

6

8

10

12

14

16

Visualizing Unsigned Addition

Wraps Around If true sum ≥ 2w

At most once

0

2w

2w+1

UAdd4(u , v)

u

v

True Sum

Modular Sum

Overflow

Overflow

4141

Mathematical Properties

Modular Addition Forms an Abelian Group Closed under addition

0 ≤ UAddw(u , v) ≤ 2w –1 Commutative

UAddw(u , v) = UAddw(v , u) Associative

UAddw(t, UAddw(u , v)) = UAddw(UAddw(t, u ), v) 0 is additive identity

UAddw(u , 0) = u Every element has additive inverse Let UCompw (u ) = 2w – u

UAddw(u , UCompw (u )) = 0

4242

Two’s Complement Addition

TAdd and UAdd have Identical Bit-Level Behavior Signed vs. unsigned addition in C:

int s, t, u, v;s = (int) ((unsigned) u + (unsigned) v);t = u + v

Will give s == t

• • •• • •

uv+

• • •u + v• • •

True Sum: w+1 bits

Operands: w bits

Discard Carry: w bits TAddw(u , v)

4343

TAdd Overflow

Functionality True sum requires

w+1 bits Drop off MSB Treat remaining

bits as 2’s comp. integer

–2w –1–1

–2w

0

2w –1

2w–1

True Sum

TAdd Result

1 000…0

1 011…1

0 000…0

0 100…0

0 111…1

100…0

000…0

011…1

PosOver

NegOver

4444

-8 -6 -4 -2 0 2 4 6-8

-6

-4-2

02

46

-8

-6

-4

-2

0

2

4

6

8

Visualizing 2’s Complement Addition

Values 4-bit two’s comp. Range from -8 to

+7 Wraps Around

If sum ≥ 2w–1

Becomes negative At most once

If sum < –2w–1

Becomes positive At most once

TAdd4(u , v)

u

vPosOver

NegOver

4545

Characterizing TAdd

Functionality True sum requires w+1

bits Drop off MSB Treat remaining bits as

2’s comp. integer

TAdd w (u,v) =u + v + 2w−1 u + v < TMin wu + v TMin w ≤ u + v ≤ TMax wu + v − 2w−1 TMax w < u + v

(NegOver)

(PosOver)

u

v

< 0 > 0

< 0

> 0

Negative Overflow

Positive Overflow

TAdd(u , v)

2w

2w

4646

Multiplication

Computing Exact Product of w-bit numbers x, y Either signed or unsigned

Ranges Unsigned: 0 ≤ x * y ≤ (2w – 1) 2 = 22w – 2w+1 + 1 Up to 2w bits

Two’s complement min: x * y ≥ (–2w–1)*(2w–1–1) = –22w–2 + 2w–1

Up to 2w–1 bits Two’s complement max: x * y ≤ (–2w–1) 2 = 22w–2

Up to 2w bits, but only for (TMinw)2

Maintaining Exact Results Would need to keep expanding word size with each product

computed Done in software by “arbitrary precision” arithmetic packages

4747

Unsigned Multiplication in C

Standard Multiplication Function Ignores high order w bits

Implements Modular ArithmeticUMultw(u , v) = u · v mod 2w

• • •• • •

uv*

• • •u · v• • •

True Product: 2*w bits

Operands: w bits

Discard w bits: w bitsUMultw(u , v)

• • •

4848

Code Security Example #2

SUN XDR library Widely used library for transferring data between

machinesvoid* copy_elements(void *ele_src[], int ele_cnt, size_t ele_size);

ele_src

malloc(ele_cnt * ele_size)

4949

XDR Code

void* copy_elements(void *ele_src[], int ele_cnt, size_t ele_size) {/** Allocate buffer for ele_cnt objects, each of ele_size bytes* and copy from locations designated by ele_src*/

void *result = malloc(ele_cnt * ele_size);if (result == NULL)

/* malloc failed */return NULL;

void *next = result;int i;for (i = 0; i < ele_cnt; i++) {

/* Copy object i to destination */memcpy(next, ele_src[i], ele_size);/* Move pointer to next memory region */next += ele_size;

}return result;

}

5050

XDR Vulnerability

What if: ele_cnt = 220 + 1 ele_size = 4096 = 212

Allocation = ??

How can I make this function secure?

malloc(ele_cnt * ele_size)

5151

Signed Multiplication in C

Standard Multiplication Function Ignores high order w bits Some of which are different for signed vs.

unsigned multiplication Lower bits are the same

• • •• • •

uv*

• • •u · v• • •

True Product: 2*w bits

Operands: w bits

Discard w bits: w bitsTMultw(u , v)

• • •

5252

Power-of-2 Multiply with Shift

Operation u << k gives u * 2k

Both signed and unsigned

Examples u << 3 == u * 8 u << 5 - u << 3 == u * 24 Most machines shift and add faster than multiply Compiler generates this code automatically

• • •0 0 1 0 0 0•••

u2k*

u · 2kTrue Product: w+k bits

Operands: w bits

Discard k bits: w bits UMultw(u , 2k)

•••

k

• • • 0 0 0•••

TMultw(u , 2k)0 0 0••••••

5353

leal (%eax,%eax,2), %eaxsall $2, %eax

Compiled Multiplication Code

C compiler automatically generates shift/add code when multiplying by constant

int mul12(int x){return x*12;

}

t <- x+x*2return t << 2;

C Function

Compiled Arithmetic Operations Explanation

5454

Unsigned Power-of-2 Divide with Shift Quotient of Unsigned by Power of 2

u >> k gives u / 2k Uses logical shift

Division Computed Hex Binary x 15213 15213 3B 6D 00111011 01101101 x >> 1 7606.5 7606 1D B6 00011101 10110110 x >> 4 950.8125 950 03 B6 00000011 10110110 x >> 8 59.4257813 59 00 3B 00000000 00111011

0 0 1 0 0 0•••u2k/

u / 2kDivision:

Operands:•••

k••• •••

•••0 0 0••• •••

u / 2k •••Result:

.

Binary Point

0

0 0 0•••0

5555

shrl $3, %eax

Compiled Unsigned Division Code

Uses logical shift for unsigned For Java Users

Logical shift written as >>>

unsigned udiv8(unsigned x){

return x/8;}

# Logical shiftreturn x >> 3;

C Function

Compiled Arithmetic Operations Explanation

5656

Signed Power-of-2 Divide with Shift Quotient of Signed by Power of 2

x >> k gives x / 2k Uses arithmetic shift Rounds wrong direction when u < 0

0 0 1 0 0 0•••x2k/

x / 2kDivision:

Operands:•••

k••• •••

•••0 ••• •••RoundDown(x / 2k) •••Result:

.

Binary Point

0 •••

Division Computed Hex Binary y -15213 -15213 C4 93 11000100 10010011 y >> 1 -7606.5 -7607 E2 49 11100010 01001001 y >> 4 -950.8125 -951 FC 49 11111100 01001001 y >> 8 -59.4257813 -60 FF C4 11111111 11000100

6060

Arithmetic: Basic Rules

Addition: Unsigned/signed: Normal addition followed by truncate,

same operation on bit level Unsigned: addition mod 2w

Mathematical addition + possible subtraction of 2w Signed: modified addition mod 2w (result in proper range) Mathematical addition + possible addition or subtraction of 2w

Multiplication: Unsigned/signed: Normal multiplication followed by truncate,

same operation on bit level Unsigned: multiplication mod 2w

Signed: modified multiplication mod 2w (result in proper range)

6161

Arithmetic: Basic Rules

Unsigned ints, 2’s complement ints are isomorphic rings: isomorphism = casting

Left shift Unsigned/signed: multiplication by 2k

Always logical shift

Right shift Unsigned: logical shift, div (division + round to zero) by 2k

Signed: arithmetic shift Positive numbers: div (division + round to zero) by 2k

Negative numbers: div (division + round away from zero) by 2k

Use biasing to fix

6262

Today: Integers


Representation: unsigned and signed Conversion, casting Expanding, truncating Addition, negation, multiplication, shifting Summary


6363

Properties of Unsigned Arithmetic

Unsigned Multiplication with Addition Forms Commutative Ring Addition is commutative group Closed under multiplication

0 ≤ UMultw(u , v) ≤ 2w –1 Multiplication Commutative

UMultw(u , v) = UMultw(v , u) Multiplication is Associative

UMultw(t, UMultw(u , v)) = UMultw(UMultw(t, u ), v) 1 is multiplicative identity

UMultw(u , 1) = u Multiplication distributes over addtion

UMultw(t, UAddw(u , v)) = UAddw(UMultw(t, u ), UMultw(t, v))

6464

Properties of Two’s Comp. Arithmetic

Isomorphic Algebras Unsigned multiplication and addition Truncating to w bits

Two’s complement multiplication and addition Truncating to w bits

Both Form Rings Isomorphic to ring of integers mod 2w

Comparison to (Mathematical) Integer Arithmetic Both are rings Integers obey ordering properties, e.g.,

u > 0 ⇒ u + v > vu > 0, v > 0 ⇒ u · v > 0

These properties are not obeyed by two’s comp. arithmeticTMax + 1 == TMin15213 * 30426 == -10030 (16-bit words)

6565

Why Should I Use Unsigned?

Don’t Use Just Because Number Nonnegative Easy to make mistakes

unsigned i;for (i = cnt-2; i >= 0; i--)a[i] += a[i+1];

Can be very subtle#define DELTA sizeof(int)int i;for (i = CNT; i-DELTA >= 0; i-= DELTA). . .

Do Use When Performing Modular Arithmetic Multiprecision arithmetic

Do Use When Using Bits to Represent Sets Logical right shift, no sign extension

6666

Today: Integers


Representation: unsigned and signed Conversion, casting Expanding, truncating Addition, negation, multiplication, shifting Summary


6767

Byte-Oriented Memory Organization

Programs Refer to Virtual Addresses Conceptually very large array of bytes Actually implemented with hierarchy of different memory types System provides address space private to particular “process” Program being executed Program can clobber its own data, but not that of others

Compiler + Run-Time System Control Allocation Where different program objects should be stored All allocation within single virtual address space

• • •

6868

Machine Words

Machine Has “Word Size” Nominal size of integer-valued data Including addresses

Most current machines use 32 bits (4 bytes) words Limits addresses to 4GB Becoming too small for memory-intensive applications

High-end systems use 64 bits (8 bytes) words Potential address space ≈ 1.8 X 1019 bytes x86-64 machines support 48-bit addresses: 256 Terabytes

Machines support multiple data formats Fractions or multiples of word size Always integral number of bytes

6969

Word-Oriented Memory Organization

Addresses Specify Byte Locations Address of first byte in word Addresses of successive

words differ by 4 (32-bit) or 8 (64-bit)

000000010002000300040005000600070008000900100011

32-bitWords Bytes Addr.

0012001300140015

64-bitWords

Addr =??

Addr =??

Addr =??

Addr =??

Addr =??

Addr =??

0000

0004

0008

0012

0000

0008

7070

Where do addresses come from?

The compilation pipeline

prog P::

foo()::

end P

P::

push ...inc SP, xjmp _foo

:foo: ...

:push ...inc SP, 4jmp 75

:...

0

75

1100

1175

LibraryRoutines

1000

175

LibraryRoutines

0

100

Compilation Assembly Linking Loading

:::

jmp 1175:...

:::

jmp 175:...

7171

int A[10];

int main() {

int j = 10;

printf("Location and difference %p %ld(1-0) %ld(1-0)\n",

&A[0],

&A[1] - &A[0],

&A[1] - A);

printf(" Int differences %ld(sizeof) %ld(1-0) %ld(2-0) %ld(3-0)\n",

sizeof(A[0]),

&A[1] - &A[0],

&A[2] - &A[0],

&A[3] - &A[0]);

printf(" Byte differences %ld(sizeof) %ld(1-0) %ld(2-0) %ld(3-0)\n",

sizeof(A[0]),

(char*)&A[1] - (char*)&A[0],

(char*)&A[2] - (char*)&A[0],

(char*)&A[3] - (char*)&A[0]);

printf(" j Value %d pointer %p\n", j, &j);

return 0;

}

7272

int A[10];

int main() {

int j = 10;

printf("Location and difference %p %ld(1-0) %ld(1-0)\n",

&A[0],

&A[1] - &A[0],

&A[1] - A);

7373

Output

int A[10];

int main() { int j = 10; printf("Location and difference %p %ld(1-0) %ld(1-0)\n",

&A[0], &A[1] - &A[0], &A[1] - A); Location and difference 0x601040 1(1-0) 1(1-0)

7474

int A[10];

int main() { … printf(" Int differences %ld(sizeof) %ld(1-0) %ld(2-0) %ld(3-0)\n",

sizeof(A[0]), &A[1] - &A[0], &A[2] - &A[0], &A[3] - &A[0]);

7575

int A[10];

int main() { … printf(" Int differences %ld(sizeof) %ld(1-0) %ld(2-0) %ld(3-0)\n",

sizeof(A[0]), &A[1] - &A[0], &A[2] - &A[0], &A[3] - &A[0]); Int differences 4(sizeof) 1(1-0) 2(2-0) 3(3-0)

7676

int A[10];

int main() { int j = 10; … printf(" Byte differences %ld(sizeof) %ld(1-0) %ld(2-0) %ld(3-0)\n",

sizeof(A[0]), (char*)&A[1] - (char*)&A[0], (char*)&A[2] - (char*)&A[0], (char*)&A[3] - (char*)&A[0]); printf(" j Value %d pointer %p\n", j, &j);

7777

int A[10];

int main() { int j = 10; … printf(" Byte differences %ld(sizeof)

%ld(1-0) %ld(2-0) %ld(3-0)\n", sizeof(A[0]), (char*)&A[1] - (char*)&A[0], (char*)&A[2] - (char*)&A[0], (char*)&A[3] - (char*)&A[0]); printf(" j Value %d pointer %p\n", j,

&j); Byte differences 4(sizeof) 4(1-0) 8(2-0) 12(3-0)

7878

int A[10];

int main() {

int j = 10;

…


return 0;

}

7979

int A[10];

int main() {

int j = 10;

…


return 0;

}

j Value 10 pointer 0x7fff860787ec

8080

Byte Ordering

How should bytes within a multi-byte word be ordered in memory?

Conventions Big Endian: Sun, PPC Mac, Internet Least significant byte has highest address

Little Endian: x86 Least significant byte has lowest address

8181

Byte Ordering Example

Big Endian Least significant byte has highest address

Little Endian Least significant byte has lowest address

Example Variable x has 4-byte representation 0x01234567 Address given by &x is 0x100

0x100 0x101 0x102 0x103

01 23 45 67

0x100 0x101 0x102 0x103

67 45 23 01

Big Endian

Little Endian01 23 45 67

67 45 23 01

8282

Address Instruction Code Assembly Rendition8048365: 5b pop %ebx8048366: 81 c3 ab 12 00 00 add $0x12ab,%ebx804836c: 83 bb 28 00 00 00 00 cmpl $0x0,0x28(%ebx)

Reading Byte-Reversed Listings

Disassembly Text representation of binary machine code Generated by program that reads the machine code

Example Fragment

Deciphering Numbers Value: 0x12ab

Pad to 32 bits: 0x000012ab

Split into bytes: 00 00 12 ab

Reverse: ab 12 00 00

8383

Examining Data Representations

Code to Print Byte Representation of Data Casting pointer to unsigned char * creates byte array

Printf directives:%p: Print pointer%x: Print Hexadecimal

typedef unsigned char *pointer;

void show_bytes(pointer start, int len){int i;for (i = 0; i < len; i++)

printf(”%p\t0x%.2x\n",start+i, start[i]);printf("\n");

}

8484

show_bytes Execution Example

int a = 15213;printf("int a = 15213;\n");show_bytes((pointer) &a, sizeof(int));

Result (Linux):int a = 15213;0x11ffffcb8 0x6d0x11ffffcb9 0x3b0x11ffffcba 0x000x11ffffcbb 0x00

8585

Data alignment

A memory address a, is said to be n-byte aligned when a is a multiple of n bytes. n is a power of two in all interesting cases Every byte address is aligned A 4-byte quantity is aligned at addresses 0, 4, 8,…

Some architectures require alignment (e.g., MIPS) Some architectures tolerate misalignment at

performance penalty (e.g., x86)

8686

Data alignment in C structs

Struct members are never reordered in C & C++ Compiler adds padding so each member is aligned

struct {char a; char b;} no padding struct {char a; short b;} one byte pad after a

Last member is padded so the total size of the structure is a multiple of the largest alignment of any structure member (so struct can go in array) struct containing int requires 4-byte alignment struct containing long requires 8-byte (on 64-bit arch)

8787

Data alignment malloc

malloc(1) 16-byte aligned results on 32-bit 32-byte aligned results on 64-bit

int posix_memalign(void **memptr, size_talignment, size_t size); Allocates size bytes Places the address of the allocated memory in *memptr Address will be a multiple of alignment, which must be

a power of two and a multiple of sizeof(void *)

8888

Representing IntegersDecimal: 15213

Binary: 0011 1011 0110 1101

Hex: 3 B 6 D

6D3B0000

IA32, x86-64

3B6D

0000

Sun

int A = 15213;

93C4FFFF

IA32, x86-64

C493

FFFF

Sun

Two’s complement representation(Covered later)

int B = -15213;

long int C = 15213;

00000000

6D3B0000

x86-64

3B6D

0000

Sun6D3B0000

IA32

8989

Representing Pointers

Different compilers & machines assign different locations to objects

int B = -15213;int *P = &B; x86-64Sun IA32

EFFFFB2C

D4F8FFBF

0C89ECFFFF7F0000

9090

char S[6] = "18243";

Representing Strings

Strings in C Represented by array of characters Each character encoded in ASCII format Standard 7-bit encoding of character set Character “0” has code 0x30

Digit i has code 0x30+i

String should be null-terminated Final character = 0

Compatibility Byte ordering not an issue

Linux/Alpha Sun

313832343300

313832343300

9191

Integer C Puzzles• x < 0 ⇒ ((x*2) < 0)• ux >= 0• x & 7 == 7 ⇒ (x<<30) < 0• ux > -1• x > y ⇒ -x < -y• x * x >= 0• x > 0 && y > 0 ⇒ x + y > 0• x >= 0 ⇒ -x <= 0• x <= 0 ⇒ -x >= 0• (x|-x)>>31 == -1• ux >> 3 == ux/8• x >> 3 == x/8• x & (x-1) != 0

int x = foo();

int y = bar();

unsigned ux = x;

unsigned uy = y;

Initialization

BITS, BYTES, AND INTEGERS · PDF fileRepresenting information as bits Bit-level manipulations...

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